Transport devices of this type make it possible to move workpieces to and from multiple processing stations disposed along the magnetic track system. The magnetic track system includes both linear and curved regions and frequently forms a closed path. Linear motors, whose primary parts are located inside a rotor, are used as drives. Multiple rotors may share the secondary part, which is arranged as a magnetic track with magnets that are lined up next to each other in a regular pattern, and can be moved independently of each other.
U.S. Pat. No. 5,642,013 describes a linear motor, which is well-suited for a transport device of the type described herein because of its especially low cogging forces. The primary part of this linear motor consists of an iron core having straight tooth faces, around which coils are wound. The secondary part includes magnets of alternating polarity, which are placed next to each other in a regular pattern. FIG. 4 of U.S. Pat. No. 5,642,013 shows this linear motor especially clearly.
PCT International Published Patent Application No. WO 2008/087130 describes a device, in which a multitude of linear primary parts is movable along magnetic tracks arranged in the form of a circular path. While the primary parts in the rotors have a straight configuration, like in a linear motor, the magnets on the circular path have a radial alignment in order to obtain a regular magnetic period along the circular path. Moreover, the placement of the magnets must be adapted for each required radius of a circular path, which involves considerable expense.
German Published Patent Application No. 10 2009 002 606 describes a transport device, which has linear as well as curved regions and whose transport path is closed on itself. The transport path includes the primary part having regularly positioned coil elements, while the rotors have magnets of alternating polarity. In this particular case, the problem of the coil placement in the linear and curved regions is addressed by providing two paths, the inner of the two paths having coil elements aligned in parallel in the linear regions, and the outer of the two paths having coil elements that have a radial alignment in the direction of the radius of curvature in the curved regions. Accordingly, a rotor has magnets that have a parallel alignment as well as magnets that have a radial alignment. However, because of the radial alignment of the magnets in the rotors, a radius of curvature is fixedly specified. The transport path with its two different tracks is also relatively complex in its manufacture.
In the essay “A Cost Effective Direct Drive Option for the Thirty Meter Telescope,” Proc. SPIE, Vol. 6273, 627335 (2006); DOI:10.1117/12.672204, a transport device based on standard components of a linear motor is described, by which a telescope having a large diameter is positioned on a circular path. Linear sections of the secondary part are sequentially placed at an angle with respect to each other, so that the sections approximate a circular path. The polarity of the magnets of the secondary part reverses itself from magnet to magnet across the sections as well. This causes interference in the periodicity of the magnetic field of the secondary part at the transitions from one section to the next, and higher cogging forces are produced in comparison with the linear scenario.